The present invention relates to the field of polymers and more particularly to the field of rigid-rod polymers.
In 1981, the U.S. Air Force sponsored research into new structural materials having low density, high strength, and high modulus. This research was reported in Macromolecules, 1981, 14, pp. 909-954. These new materials were formed by condensing tetrafunctional benzene derivatives with aromatic p-dicarboxylic acids to give heteroaromatic rings. The resulting polymers are known as rigid-rod polymers. One rigid-rod polymer, polybenzothiozole (PBT), is formed by the condensation of 2,5-diamino-1,4-benzenedithiol with terephthalic acid in polyphosphoric acid. Another, polybenzoxazole is formed by the condensation of 4,6-diamino-1,3-benzenediol with terephthalic acid. A third, polybenzimidazole is formed by the condensation of 1,2,4,5-tetraaminobenzene with terephthalic acid. Rigid-rod polymers were found to exhibit liquid crystalline behavior in solution, suggesting potential uses as nonlinear optical materials or in molecular composites. Several theoretical and structural studies on these materials have been published. See for example, Wolfe, J. F. "Polybenzothiazoles and Oxazoles", Encyclopedia of Polymer Science and Engineering 1988, 11, 601.
The high intermolecular attraction and degree of molecular order of rigid-rod polymers, which results in their excellent mechanical properties, also makes these materials insoluble in everything except strong acids. Dissolution of rigid rods in acid occurs by protonation of the heteroatoms but is limited by charge delocalization along the chain resulting in fewer protonations. Processing is therefore difficult. Attempts have been made to improve the solubility characteristics by inserting pendant groups on the polymer chain, or small numbers of flexible groups into the chain. These changes usually resulted in improved processability at the expense of the thermal stability and mechanical properties.
Rigid-rod polymers could potentially be useful as molecular reinforcement for composites or for optical applications, for example aircraft windshields. Molecular reinforcements are similar to macroscopic chopped-fiber reinforcements. The advantage of molecular reinforcements is that interfacial adhesion problems are minimized or eliminated so that fracture toughness, impact resistance and dimensional stability are improved. The use of rigid-rod polymers for optical applications has been hampered by the inherently dark colors of the polymers which result from their highly conjugated backbones.
If a method could be found to reduce the high intermolecular interactions of rigid-rod polymers it would be a great advance in the field. Reducing the intermolecular attraction would add disorder to the rods, permit greater relative protonation, enhance processability and, by reducing conjugation of the aromatic-heteroaromatic rings, would reduce the color of the rigid rod polymers.